A good place to rest, this fold of shade at the arroyo's steep edge, and a good place to ponder. Two days ago, a July cloudburst dropped an inch of rain here in Arizona's Saguaro National Park. Water is still seeping down the arroyo and collecting in pools. Bright green algae already coat the bottoms. Mourning doves, cactus wrens, and hundreds of bees sip from the edges. And somehow, among these mountainsides of stones too hot to touch, tadpoles have materialized within the pools. Native Americans who paused beneath the same rock wall long ago left paintings of humans, creatures, and spirits. If, as it seems, the ﬁgures are keeping watch, they have witnessed many times before the drama of survival that is about to play out.

Many frogs and toads need months to develop from eggs into adult form. Spadefoots, which are similar to true toads but have smooth skin, catlike eyes, teeth, and a sharp nail on the hind feet, live by different rules. Adults exist in a dormant condition buried beneath the desert floor like plant bulbs until they sense vibrations from thunder and raindrops. Then they emerge to gather at runoff pools, send ear-splitting choruses into the night, and breed. With metabolisms set on fast-forward, the offspring can transform into miniature grown-ups in as few as eight days. The race is on to become an air-breather hopping about and fattening on insects before the pond dries, leaving the moist-skinned amphibians no choice except to spade back underground and wait for another heavy rain. That might be months. It might be more than a year.

If the tadpoles in the arroyo pools are red-spotted toads, usually found around more permanent water, they're doomed. If they're spadefoots, they have a chance. Every day, they grow a little larger. Every day, the pools are smaller, vaporizing under the midday sun. . . .

"A year ago this month, she was on death’s door. Skin draped over a skeleton. Out moving all the time, got to ﬁnd food. But there was none," says Jon Davis, a graduate student from Arizona State University. "I wanted to take her in, I was so sure she would die." In the light of our headlamps, a 15-inch Gila monster, ﬁrmly held behind the jaw, dangles like a toy dragon covered with pink and black beadwork. The belly is wide, the wriggling tail plump as Polish sausage. We can see baby rabbit fur stuck to the mouth. Life for this reptile is plainly on the upswing. Davis pulls a couple of cactus spines from the female’s toes and puts her in a bag for closer inspection by daylight.